Abstract: By analyzing an abnormal case of the secondary circuit of a 500kV substation , it was found that if the grounding point of the secondary circuit of the voltage transformer is in the protection room, once a single-phase grounding occurs in the secondary circuit of the voltage transformer, a short-circuit circulating current will appear in the relevant secondary circuit. This circulating current will cause a potential shift of N600 to ground in other voltage secondary circuits, resulting in abnormal voltage collection of other operating device and an alarm signal. Finally, it was concluded that independent secondary circuits without electrical connection with other voltage transformer secondary circuits should be grounded locally at one point in the switch yard.